Using rigify armature, how do I create a control for eye color?
You can add any object to be the controller. Then add drivers to the RGB component of your material. (Right click the RGB color box and click Add Driver). Then for the eyes, go into the Drivers Editor and select the Drier to be of type Averaged Value. For the object, have it be the object you want to control the color. Set type to Y rotation. You can do this for R G B and A. Then when you move/rotate your Nurbs circle (or whatever object you want, the color will change (need to be in rendered view).

Here is a .blend file with a simple set up. If this doesn't quite make sense, let me know!